Non-pneumatic deformable structures for use in the support of loads under rolling conditions, such as to support loads for automotive vehicles have been described, for example in U.S. Pat. No. 7,201,194, which is commonly owned by the assignee of the current invention and is incorporated by reference in its entirety. The structurally supported, non-pneumatic tire disclosed therein includes an annular band that supports the load, a plurality of web spokes that transmit in tension the load forces between the annular band and a wheel or hub. A particularly useful design for the web spokes that improves spoke durability and allows the designer to vary the initial stiffness of vertical load versus vertical deflection is disclosed in United States Patent Publication No. 2009/0294000A1, which is also commonly owned by the assignee of the current invention and is incorporated by reference in its entirety. The non-pneumatic article supports its load solely through the structural properties and, contrary to the mechanism in pneumatic tires, without support from internal air pressure. In further embodiments, the annular shear band may comprise one or more sets of reinforcements that are radially spaced apart by an elastomeric material to form a shear layer between sets of reinforcements. In a particular embodiment, the shear band comprises a first membrane adhered to the radially inward extent of the shear layer and at least a second membrane adhered to the radially outer extent of the shear layer.
The non-pneumatic deformable structure may be manufactured by several alternative means. In one exemplary method the shear layer and web spokes may be made of the same material, for example polyurethane, and can be manufactured by a molding process to produce the non-pneumatic article. An alternative means comprises manufacturing the annular shear band as a separate article, and then forming the complete article by molding the spokes in a mold that uses the annular band as the radially outer surface and the hub as the radially inner surface. This method allows the designer to specify different materials for the annular band and for the web spokes. For the exemplary article disclosed in U.S. Pat. No. 7,201,194, the annular band comprises a rubber shear layer and includes an outer tread portion that is constructed and then vulcanized as a unit by conventional means known in the tire industry. The annular band and the hub are placed in a mold, where the mold cores and profiles define the geometry of the spokes for the finished article.
A common molding technique used for articles having axisymmetric elements (the annular band and the hub) and essentially radial elements (the web spokes) is to use a centrifugal casting method where the mold rotates at a given rotation speed and the material to be molded is poured into the mold near the axis of rotation of the mold. In an exemplary method, a two-pail polyurethane elastomer is premixed, and then poured into the mold, allowed to cure, and then the finished article may be removed from the mold. The centripetal force created by the rotation of the mold produces a radial pressure gradient component in the liquid elastomer that facilitates complete filling of the mold to avoid molding deficiencies in the finished article. For an example where the mold rotates about a vertical axis of rotation, the centripetal pressure gradient component combines with a gravitational pressure gradient component.
In such a process it is not unexpected for air to be entrapped either in the premixed polyurethane or in small cavities within the mold. In either case, the entrapped air can form small bubbles which negatively affect the aesthetic appearance or, possibly, the fatigue durability of the finished article. The small air bubbles have a density which is on the order of one thousand times less than the density of the polyurethane. Under the actions of the centripetal and gravitational pressure gradients, the bubbles are subject to a buoyancy force that tends to cause the bubbles to migrate to the axially upper edge of the interface between the spoke element and the annular band. The finished article will then contain small voids at this location which may negatively affect the fatigue durability of the finished article.
Accordingly, there is a need to solve the problem of voids in the finished non-pneumatic article and to improve the performance of the article. A new design of the molded article and the corresponding mold is thereby disclosed that urges the entrapped air bubbles to migrate towards a location where they can be vented, thereby eliminating the voids in the finished article.